A year ago I was given a calla lily. In the fall I cut it down and put it in the cool, dark basement and just left it until spring. Brought it out in the spring and it was beautiful. It has grown large. I would like to split it. Should I do that before I put it in the basement or wait until I bring it back out? So, before it goes to sleep or after it wakes up?
I’d suggest dividing your plant before it goes to sleep for the winter (it’s now late September).
The usual way to over-winter the calla lily is to dig up the plant’s tuber-like rhizome (the stem that grows underground, which the plant uses to store energy). This is generally done after frost has killed the foliage (to ensure that the plant has stored nutrients it requires to survive the winter) and brush off any soil attached to it. Keep it in a warm, dry spot to dry for up to a week, then store it in a paper bag (one that contains peat moss or vermiculite if you wish) or wrapped in newspaper in a cool dry place (no lower than 10 degrees C or 50 degrees F). From the information you provided, you seem to have simply cut back your plant at the end of the summer and over-wintered it (in its pot) in the basement, which was a great success.
Dividing the calla rhizome is helpful once the plant starts to decline – about every 3-5 years. If you feel that now’s the time to divide the plant, this can be done once it finishes blooming. Lift the rhizome as described above and cut it into sections, making sure each has at least one eye (a growing point). Allow the rhizomes to dry for a day or so – this permits formation of a callous over the cut, which will help prevent disease. To prevent rot and fungus, some experts also recommend sprinkling the bulbs with sulfur dust prior to storage. Then store as above and replant them in the spring.
All the best with your lovely plant!